1. Field of the Invention
This invention relates to a light measuring control system for single lens reflex camera which makes it possible to select a desired sensitivity measuring pattern.
2. Description of the Prior Art
In general, the light measuring systems built into the single lens reflex cameras are to perform either an average measuring function using the light coming from the overall field of view, or a spot measuring function using the light coming from the central part of the viewing field, or a centerweighted measuring function having primary emphasis on the central area of the image.
The average measuring method when applied to a subject of uniform brightness distribution or a subject having small difference between the brightness of the center and surrounding thereof can give a correct exposure.
However, when the brightness of the subject differs greatly from part to part, false readings are produced with the averaged-light measure and one must employ either the spot measure or the centerweighting measure to obtain a proper exposure.
Previously, a system using two photosensitive elements, one which is positioned to detect light from the central part of the whole area of an image on the focusing screen and another which is positioned to detect light from the rest or marginal part of the image was proposed. When the difference between the outputs of both the photosensitive elements, or between the intensities of light is larger than a predetermined level, one of the photosensitive elements is automatically selected by an electronic circuit and connected to an exposure control circuit, and when smaller than the predetermined level, the other element is selected automatically.
Since the conventional system is operated with automatic selection of the average measuring mode and either the spot or the centerweighted measuring mode, depending upon whether or not the subject has an average distribution of light, the expert photographer is unable to fully control the camera setting and thus is not satisfied with the apparatus. Different subject tones require different light measuring modes, making it difficult for beginners to read the corrected exposure because it is necessary to take into account the characteristics of the individual measuring modes.
The conventional method of light measurement in automatic response to a difference in brightnesses of central and marginal areas of an image results in the following problem. Photographic situations where the tones of a scene differ from place to place, may be mainly classified into two cases. In the first case, the scene is light as a whole with the brightest highlight lying particularly at the center. In the second case, the scene is dark over the entire area except in the central area of bright luminance.
The conventional control system for selection of the measuring modes causes setting of the centerweighted, or spot measuring mode in response to these cases. In the first case, good exposure results can be obtained. But, in the second case, an accurate measuring cannot be always assured. For example, in evening and night photography, when a road lamp or a car's head light lies in the central area of the image, with a prior art camera switched to the centerweighted or spot measuring mode, the arrangement of the photosensitive elements lays an extremely heavy emphasis on the street lamp or head light, thus leaving a high possibility of occurrence of not only improper exposure but also an improper exposure that cannot be saved by any later processing.
Furthermore, in the camera provided with the conventional light measuring system, the automatic exposure mode and the manual exposure mode are made selectable. When in the automatic exposure mode, exposure is automatically controlled based on the output of the light measure. In the manual exposure mode, the dislayed reading is taken as a reference and one of the f-stop and shutter-speed combinations for an equivalent exposure is chosen manually.
In the conventional camera, however, when switched to the manual exposure mode, the display of the light value is the same as when in automatic exposure. Therefore, the selected f-stop and shutter-speed combination of the camera must be determined by the experience of the photographer. An inexperienced photographer may read that combination inaccurately and correct exposure cannot be made. This tendency is more pronounced with users who have little experience.